tg3: Make the TX BD DMA limit configurable

The 57766 ASIC rev will impose a new TX BD DMA limit on the driver.
This patch prepares for 57766 support by making the tx BD DMA limit
tunable.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>